The present invention relates to an objective optical system for an optical information recording/reproducing apparatus configured to record information to and/or reproduce information from a plurality of types of optical discs based on different standards, and to an optical information recording/reproducing apparatus on which such an objective optical system is mounted.
There exist various standards of optical discs, such as DVD (Digital Versatile Disc) and BD (Blu-ray Disc), differing in recording density, protective layer thickness, etc. Therefore, an objective optical system mounted on the optical information recording/reproducing apparatus is required to have a compatibility with a plurality of types of optical discs. In this case, the term “compatibility” means to guarantee realizing information recording and information reproducing without the need for replacement of components even when the optical disc being used is changed.
In order to have the compatibility with the plurality of types of optical discs based on the different standards, it is necessary to correct the relative spherical aberration which is caused depending on the difference in protective layer thickness between the optical discs and to form a suitable beam spot in accordance with the difference in recording density between the optical discs by changing the numerical aperture NA of the objective optical system. The optical information recording/reproducing apparatus is configured to use a plurality of types of laser beams having different wavelengths respectively for the plurality of recording densities of the optical discs. The optical information recording/reproducing apparatus uses, for example, light having the wavelength of approximately 790 nm (i.e., so-called near infrared laser light) for information recording or information reproducing for CD, light having the wavelength of approximately 660 nm (i.e., so-called red laser light) for information recording or information reproducing for DVD and light having the wavelength of approximately 405 nm (i.e., so-called blue laser light) for information recording or information reproducing for BD. Japanese Patent Provisional Publication No. 2009-199707A (hereafter, referred to as patent document #1) discloses a configuration of an optical information recording/reproducing apparatus having the compatibility with the three types of optical discs.